Re: [boost] More type_traits confusion

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2001-01-21 20:31:19

David Abrahams wrote on 1/21/2001 7:55 PM
>Now I've been using is_signed_integral<T> to find out whether a
>compiler-supplied type is signed or not, and I've been getting away with it,
>but it turns out you can't even do that.
>is_standard_signed_integral<T> isn't specialized for char. I (now)
>understand the logic behind it, since the standard doesn't specify whether
>char is signed or unsigned, but then why do all my uses of
>is_signed_integral<char> seem to compile without complaint?
>
>I think we have to get a lot clearer about the intent of type_traits in its
>documentation.

Instead of the current is_signed_XXX structs I prefer a simpler struct 
that cuts across all XXX:

template <class T> struct is_signed
        {static const bool value = T(-1) < T(0);};

template <class T> struct is_unsigned
        {static const bool value = !is_signed<T>::value;};

Requires that T is constructable from the literals -1 and 0.  
Compile-time results if that requirement is not met (and thus signedness 
is not likely to have meaning for that type).

Note that is_signed<char>::value is both well formed and well-defined, 
though its result will vary with implementations.

-Howard